1. Field of the Invention
The present invention relates to an MVA (Multi-domain Vertical Alignment) mode liquid crystal display device and a method of manufacturing the MVA mode liquid crystal display device. The present invention relates particularly to a liquid crystal display device in which polymers for determining directions in which liquid crystal molecules tilt are formed in a liquid crystal layer, and to a method of manufacturing the same.
2. Description of the Prior Art
In general, a liquid crystal display device is configured of a liquid crystal panel and polarizing plates. The liquid crystal panel is fabricated to contain liquid crystal between two substrates. The polarizing plates are arranged respectively in the two sides of the liquid crystal panel. A picture element electrode is formed in each of picture elements in one substrate of the liquid crystal panel. A common electrode used commonly for the picture elements is formed in the other substrate of the liquid crystal panel. When voltage is applied between the picture element electrode and the common electrode, alignment orientations of liquid crystal molecules change depending on the voltage. As a result, this changes an amount of light which passes through the liquid crystal panel and the polarizing plates arranged respectively on the two sides of the liquid crystal panel. Control of the applied voltage for each of the picture elements makes it possible to display a desired image to be displayed on the liquid crystal display device.
With regard to a TN (Twisted Nematic) mode liquid crystal display device which has been heretofore used widely, liquid crystal with positive dielectric anisotropy is used, and the liquid crystal molecules are twisted and aligned between the two substrates. However, the TN mode liquid crystal display device has a disadvantage of having insufficient viewing angle characteristics. In other words, with regard to the TN mode liquid crystal display device, tone and contrast are extremely deteriorated when the liquid crystal panel is viewed in an oblique direction. Accordingly, the contrast is reversed in extreme cases.
An IPS (In-Plane Switching) mode liquid crystal display device and an MVA (Multi-domain Vertical Alignment) mode liquid crystal display device have been known as liquid crystal display devices which are good at viewing angle characteristics. In the case of the IPS mode liquid crystal display device, picture element electrodes shaped each like a line and common electrodes each shaped like a line are arranged alternately in one of the two substrates. If voltage is applied between the picture element electrodes and the common electrodes, orientations respectively of the liquid crystal molecules change in a plane parallel with the surface of the substrate depending on the voltage.
Although, however, the IPS mode liquid crystal display device is good at viewing angle characteristics, the orientations respectively of the liquid crystal molecules over the picture element electrodes and the common electrodes cannot be controlled. That is because the voltage is applied in a direction parallel with the surface of the substrate. This brings about a disadvantage that the substantial aperture ratio of the IPS mode liquid crystal display device is low, and that the screen is accordingly dark if a powerful backlight is not used.
In the case of the MVA mode liquid crystal display device, picture element electrodes are formed in one of the two substrates, and a common electrode is formed in the other of the two substrates. In addition, with regard to a generally-used MVA mode liquid crystal display device, bank-shaped protrusions made of a dielectric material extending in oblique directions are formed on the common electrode. Each of the picture element electrodes is provided with slits parallel with the protrusions.
In the case of the MVA mode liquid crystal display device, while no voltage is being applied, the liquid crystal molecules are aligned in a direction perpendicular to the substrates. When voltage is applied between the picture element electrode and the common electrode, the liquid crystal molecules are aligned to tilt at an angle corresponding to the voltage. At this time, a plurality of domains are formed in each of the picture element by the slits provided in the picture element electrode and bank-shaped protrusions. The directions in which the liquid crystal molecules tilt vary from one domain to another. Formation of the plurality of domains in each of the picture elements where the directions in which the liquid crystal molecules tilt vary from one domain to another makes it possible to obtain good viewing angle characteristics.
In the case of the aforementioned MVA mode liquid crystal display device, however, the slits and the protrusions decrease the substantial aperture ratio. Accordingly, the substantial aperture ratio of the MVA mode liquid crystal display device is lower than that of the TN mode liquid crystal display device, although the substantial aperture ratio of the MVA mode liquid crystal display device is not so low as that of the IPS mode liquid crystal display device. For this reason, the MVA mode liquid crystal display device needs a powerful backlight. As a result, this kind of MVA mode liquid crystal display device has hardly been adopted for a notebook personal computer, which requires power consumption to be lower.
Japanese Patent Laid-open Official Gazette No. 2003-149647 has disclosed an MVA mode liquid crystal display device which was developed in order to solve the aforementioned problems. FIG. 1 is a plan view showing the MVA mode liquid crystal display device. Incidentally, FIG. 1 shows two picture element regions.
A plurality of gate bus lines 11 extending in the horizontal direction (X-axis direction) and a plurality of data bus lines 12 extending in the vertical direction (Y-axis direction) are formed on one of the two substrates constituting a liquid crystal panel. Insulating films (gate insulating films) are formed between a group of gate bus lines 11 and a group of data bus lines 12. The insulating films electrically isolate the group of gate bus lines 11 from the group of data bus lines 12. Each of the rectangular areas defined by the gate bus lines 11 and the data bus lines 12 is a picture element region.
A TFT (thin film transistor) 14 and a picture element electrode 15 are formed in each picture element region. As shown in FIG. 1, the TFT 14 uses a part of the gate bus line 11 as a gate electrode. A semiconductor film (not illustrated) which functions as an activation layer of the TFT 14 is formed over the gate electrode. In addition, a drain electrode 14a and a source electrode 14b are connected respectively to the two sides of this semiconductor film in the Y-axis direction. The source electrode 14b of the TFT 14 is electrically connected to the data bus line 12, and the drain electrode 14a is electrically connected to the picture element electrode 15.
In this patent application, it should be noted that, out of the two electrodes connected to the semiconductor film which functions as the activation layer of the TFT, one electrode connected to the data bus line is termed as a source electrode, and the other electrode connected to the picture element electrode is termed as a drain electrode.
The picture element electrode 15 is formed, for example, of a transparent conductive material such as ITO (Indium-Tin Oxide). Slits 15a are formed in this picture element electrode 15 in order to cause liquid crystal molecules to be aligned in four directions when voltage is applied. In other words, the picture element electrode 15 is divided into four domain controlling regions with the center line in parallel with the X-axis and the center line in parallel with the Y-axis defined as boundaries. A plurality of slits 15a extending in a direction at an angle of approximately 45 degrees to the X axis are formed in a first region (upper right region). A plurality of slits 15a extending in a direction at an angle of approximately 135 degrees to the X axis are formed in a second region (upper left region). A plurality of slits 15a extending in a direction at an angle of approximately 225 degrees to the X axis are formed in a third region (lower left region). A plurality of slits 15a extending in a direction at an angle of approximately 315 degrees to the X axis are formed in a fourth region (lower right region). A vertical alignment film (not illustrated) made of polyimide is formed on the picture element electrode 15.
Black matrices, color filters and a common electrode are formed in the other substrate. The black matrices are made, for example, of a metal such as Cr (chromium), or of a black resin. The black matrices are arranged respectively in positions, which are opposite to the gate bus lines 11, the data bus lines 12 and the TFTs 14. The color filters are classified into three types, such as red, green and blue. Any one of the three types of color filters is arranged in each of the picture elements. The common electrode is made of a transparent conductive material such as ITO, and is formed on the color filters. A vertical alignment film made of polyimide is formed on the common electrode.
A liquid crystal panel is constituted in the following manner. These two substrates are arranged opposite to each other with spacers (not illustrated) interposed between the two substrates. Liquid crystal with negative dielectric anisotropy is filled in the space between the two substrates. Hereinafter, out of the two substrates constituting the liquid crystal panel, one substrate on which TFTs are formed will be termed as a TFT substrate, and the other substrate which is arranged opposite to the TFT substrate will be termed as an opposing substrate.
In the case of the MVA mode liquid crystal display device shown in FIG. 1, while no voltage is being applied to the picture element electrode 15, the liquid crystal molecules are aligned virtually perpendicularly to the surface of the substrate. While voltage is being applied to the picture element electrode 15, the liquid crystal molecules 10 tilt in directions in which the respective slits 15a extend as schematically shown in FIG. 1. Accordingly, four domains are formed in each picture element where the directions in which the liquid crystal molecules 10 tilt vary from one domain to another. This secures good viewing angle characteristics.
Changing the subject, in the case of the MVA mode liquid crystal display device shown in FIG. 1, it remains not to be determined whether the liquid crystal molecules 10 tilt inwards (in directions towards the center of the picture element) or outwards (in directions towards the outsides of the picture element), immediately after voltage is applied to the picture element electrode 15. First of all, the electric field generated from edges of the picture element electrode 15 determines that the liquid crystal molecules 10 in the edges of the picture element electrode 15 (near the data bus line 12) tilt inwards. Subsequently, directions in which the liquid crystal molecules 10 tilt propagate towards the center of the picture element. Accordingly, it takes a long time for all the liquid crystal molecules 10 in a picture element to complete tilting in predetermined directions. This brings about a problem that the response time is long.
Japanese Patent Laid-open Official Gazette No. 2003-149647, which has been mentioned above, disclosed that a liquid crystal display device is fabricated in the following manner. First, liquid crystal to which polymerizable components (monomers) are added is filled into the space between the pair of substrates. Then, voltage is applied between the picture element electrode and the common electrode, thereby causing the resultant liquid crystal to tilt in predetermined directions. Thereafter, beams of ultraviolet light are irradiated on the resultant liquid crystal, and thereby the polymerizable components are polymerized. By this, polymers are made in the liquid crystal layer. In the case of the liquid crystal display device thus manufactured, the polymers in the liquid crystal layer determine directions in which the liquid crystal molecules tilt. For this reason, no sooner is voltage applied between the picture element electrode and the common electrode than all of the liquid crystal molecules in the picture element start to tilt in predetermined directions. Accordingly, the response time is reduced to an extreme extent.
In the process of manufacturing liquid crystal display devices shown in FIG. 1, when polymerizable components added to liquid crystal are polymerized, a voltage (for example, 20V) higher than a voltage (usually, approximately 4 to 6 V) which is applied between picture element electrodes and common electrodes while the liquid crystal display devices are being actually used has heretofore been applied between the picture element electrodes and the common electrodes. That is because the work efficiency is attempted to be enhanced by shortening time needed for liquid crystal molecules to complete tilting in predetermined directions. Through experiments and studies, however, the present applicants have made the following findings. If a higher voltage is applied to the liquid crystal quickly, liquid crystal molecules which are going to tilt in directions different from one another are present in a single domain controlling region at a time. Accordingly, this causes disturbance of the alignment (what is termed as “disclination”). If the polymers are formed by irradiating beams of ultraviolet light on the liquid crystal molecules while the liquid crystal molecules are being in this condition, the liquid crystal molecules are disturbed, too, while the liquid crystal display device is being actually used. This presents a cause of deteriorating the display quality.
In addition, a further enhancement in the display quality has been expected for liquid crystal display devices in recent years. In general, in the case of a vertical alignment (VA) mode liquid crystal display device, it has been known that a T-V characteristic (transmittance-applied voltage characteristic) to be observed when a liquid crystal display device is viewed from the front is different from that to be observed when the liquid crystal display device is viewed in an oblique direction. The MVA mode liquid crystal display devices also have a similar defect. FIG. 2 is a diagram showing a T-V characteristic to be observed when an MVA mode liquid crystal display device is viewed from the front, and a T-V characteristic to be observed when the MVA mode liquid crystal display device is viewed in a direction at an azimuth angle of 90 degrees and at a polar angle of 60 degrees (in a direction downwards diagonally). Incidentally, in FIG. 2, the axis of abscissa represents 256 gray scales into which the gradation from black to white is divided. Each of the gray scales corresponds to a voltage applied to a picture element electrode. The larger a value on the gray scale is, the higher the voltage applied to the picture element electrode is. Furthermore, in FIG. 2, a transmittance is denominated in a value (T/Twhite) relative to the transmittance (Twhite) which is defined as 1 (one) while white is being displayed.
As understood from FIG. 2, in the case of the conventional MVA mode liquid crystal display device, the T-V characteristic to be observed when the liquid crystal display device is viewed from the front is much different from that to be observed when the liquid crystal display device is viewed in the oblique direction. For this reason, the conventional MVA mode liquid crystal display device has a disadvantage that the display quality is deteriorated when viewed in an oblique direction although a preferable display quality can be obtained when viewed from the front. In particular, as understood from FIG. 2, in the case of the conventional MVA mode liquid crystal display device, the line representing the T-V characteristic to be observed when the liquid crystal display device is viewed in the oblique direction undulates to a large extent in comparison with the line representing the T-V characteristic to be observed when the liquid crystal display device is viewed from the front. Accordingly, when middle gray-scales are displayed, the difference in brightness becomes smaller between the viewing from the front and the viewing in the oblique direction. For this reason, a phenomenon occurs in which an image to be viewed in the oblique direction looks whitish (washes out) in comparison with an image to be viewed from the front, thus causing the display quality to be deteriorated. Moreover, an anisotropy in terms of a refractive index of liquid crystal has wavelength dependency. For this reason, it is likely that color to be seen when the conventional MVA mode liquid crystal display device is viewed from the front may be different from that to be seen when the conventional MVA mode liquid crystal display device is viewed in an oblique direction in some cases.